Add SVG export for cells and libraries (#192)

## Summary

- Add `cell_to_svg` and `library_to_svg` public functions to the `gdsr`
crate
- Render polygons, paths, boxes, text, and nodes as SVG elements with
layer-based coloring (same 16-color palette as viewer)
- Flatten cell references recursively so nested hierarchies render
correctly
- Auto-size viewport to bounding box with 5% margin
- Flip Y axis (GDS Y-up → SVG Y-down) via SVG transform group
- Escape text content for XML safety

Closes #134

## Test plan

- 11 new snapshot tests covering each element type, reference
flattening, multi-layer colors, XML escaping, empty cells, and Y-flip
transform
- Full test suite passes (691 tests)
- All pre-commit checks pass

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: MatthewMckee4

crates/gdsr/examples/sample.rs

@@ -1,6 +1,6 @@
 use gdsr::{
     Cell, DataType, Grid, HorizontalPresentation, Layer, Library, Path, PathType, Point, Polygon,
-    Reference, Text, Unit, VerticalPresentation,
+    Reference, Text, Unit, VerticalPresentation, cell_to_svg,
 };
 
 fn main() -> Result<(), gdsr::GdsError> {
@@ -155,5 +155,12 @@ fn main() -> Result<(), gdsr::GdsError> {
 
     library.write_file("sample.gds", 1e-6, 1e-9)?;
 
+    let svg = cell_to_svg(
+        library.get_cell("top").expect("top cell exists"),
+        &library,
+        1e-9,
+    );
+    std::fs::write("sample.svg", &svg).expect("failed to write SVG");
+
     Ok(())
 }

crates/gdsr/src/lib.rs

@@ -27,3 +27,4 @@ pub use traits::{Dimensions, Movable, ToGds, Transformable};
 pub use transformation::{Reflection, Rotation, Scale, Transformation, Translation};
 pub use types::{AngleInRadians, DataType, Layer, LayerMapping};
 pub use units::{DEFAULT_FLOAT_UNITS, DEFAULT_INTEGER_UNITS, FloatUnit, IntegerUnit, Unit};
+pub use utils::svg::cell_to_svg;

crates/gdsr/src/utils/mod.rs

@@ -1,2 +1,3 @@
 pub mod gds_format;
 pub mod io;
+pub mod svg;